In recent years, much attention has been paid to devices employing tunneling junctions. A tunneling junction consists of two electrodes separated by a vacuum, gas or liquid gap on the order of 1 nm. Junctions consisting of gold spheres suspended on piezo-electric positioners have been used to demonstrate tunneling. Scanning tunneling microscopes with atomically sharp translating scanning tips spaced from an object being examined by piezo-electric positioners are relatively commonly used. The art also shows "squeezable" electron tunneling junctions and "break" junctions, both employing electromagnetic positioners. Thus, it is apparent that both electromagnetic and piezo-electric devices have been used to control the spacing of conductors in mechanically adjustable tunneling junctions.
As indicated, a tunneling junction in essence comprises two conductors separated by a very small gap on the order of 1 nm. If a small potential is provided across the electrodes, a "tunneling current" flows, even in the absence of physical contact, between the two portions of the junction. The tunneling junction resistance varies exponentially with the distance. Therefore, the junction resistance is a very sensitive indicator of variations in the electrode spacing. This known fact is employed by the present invention.
Most uses of mechanically adjustable tunneling junctions to date have related to scanning tunneling microscopy (STM). In this case, an atomically sharp tunneling electrode is traversed over a conductive substrate to be imaged. As noted, the junction resistance and thus tunneling current vary with electrode-to-substrate spacing. The current is monitored, and the relative height of the electrode from the nominal plane of the substrate is varied in a servo loop so as to maintain the current and thus the spacing constant. Variations in the servo signal are thus responsive to variations in the surface topography. Therefore, the servo signal can be used to image features of atomic size on the surface of the planar electrode. To date, STM devices have typically employed piezo-electric positioners for control of the electrode spacing. See Binnig et al, Appl. Phys. Lett., 40, 178 (1982).
Squeezable electron tunneling junctions are disclosed by Moreland et al in "Electro-Magnetic Squeezer for Compressing Squeezable Electron Tunneling Junctions," Review of Scientific Instruments, 55(3), 1984 and in "Squeezable Electron Tunneling Junctions," Applied Physics Letters, 43(4), 1983. In these devices an electromagnet was used to control the gap spacing. An electromagnet was also used to control the gap spacing in so-called "break" junctions described also by Moreland et al in "Electron Tunneling Experiments using Nb-Sn `break` junctions," Journal of Applied Physics, 58(10), (1985). In the latter work, a conductive filament was mounted on a flexible beam and broken to form very closely spaced surfaces of similar shape. The spacing of the junction, and thus the resistance of the tunneling junction, was controlled by varying the bending of the flexible beam. This work was performed using an Nb-Sn filament at cryogenic temperatures such that the filament itself was superconductive. However, nonsuperconductive filaments are substantially equally useful. An electromagnet was used to control the electrode spacing in a servo loop in which the current in the electromagnet was the control variable.
The present inventors are aware of no publication in which tunneling junctions controlled by an electromagnet have been used for force detection per se. Related devices referred to as atomic force microscopes (AFM's) have been used for force detection. In an AFM, a tunneling junction can be used to detect the motion of a flexible cantilever as it responds to an applied interatomic force. Variations in the tunneling current can be detected and used to image variations in the force on the cantilever as it is scanned over a sample's surface using a piezoelectric positioner. Alternatively, the force can be kept constant by disposing the cantilever in a feedback loop. Tunnel junction gap measurements of the cantilever deflection as above can be replaced by interferometric or capacitive detection schemes. However, it will be appreciated that the fact that the force is applied to the cantilever and its displacement measured by a second device unduly complicates such schemes for force detection.